This invention relates to the press bending and tempering of glass sheets and particularly relates to the press bending of glass sheets of relatively thin dimensions such as 3 and 4 millimeters thickness. Glass sheets that are exceedingly thin are difficult to shape and temper, because thin sheets often crack due to excessive cooling before shaping with equipment designed for thicker glass. Furthermore, if the glass is heated to an elevated temperature at which it is very soft, it is very difficult to maintain the shape of the glass and to avoid indentation by tongs which grip the glass during its transfer from a furnace where the glass is heated to its deformation temperature to a shaping station where the glass sheet is engaged between complementary shaping molds of the press bending type and the cooling station where the glass is suddenly quenched on its major surfaces while the interior portion of the glass remains in a fluid condition. Unless the glass is heated within a narrower temperature range than that permitted for thicker glass and/or transferred from the furnace to the cooling station more rapidly than required for thicker glass, the glass will either lose its desired shape if heated too hot initially or become too cool to develop a sufficient temper if not heated initially to a sufficiently high temperature to compensate for its cooling during its conveyance to the cooling station.
Press bending molds for shaping glass sheets suspended from tongs are provided with slots in their upper edges to receive glass gripping tongs. These slots should be sufficiently large to enable the tongs to be received therein when the press bending molds close to engage a glass sheet for shaping. The tong clearance slots should be made as small as possible to provide maximum control for the shape the molds impress on the tong-gripped glass sheet during shaping by press bending.
When thin glass sheets are shaped between complementary shaping molds, the glass shaping molds sometimes impart a force on the tongs through the thin glass that causes the tongs to twist or tilt away from the shaped glass. Such tong twisting or tilting causes the glass to become distorted from its bend desired. Damage to tongs or molds used to process glass sheets may result from misalignment between tong clearance slots provided along the upper edge of the shaped press bending molds during the engagement of the shaped molds against the opposite major surfaces of a flat glass sheet when the tongs engage the molds instead of being received within the tong clearance slots. Extending the width of the slots reduces the likelihood of tong or mold damage due to direct mold contact by a tilted tong. However, widening the slots decreases the ability of the molds to control the glass sheet shape in the vicinity of the tong clearance slots. Other glass defects due to twisting or tilting of the tongs have been moderated to some extent for molds that engage a glass sheet at rest to shape the latter. However, no molds that move synchronously with a glass sheet through a shaping station have been able to consistently avoid the glass defects due to tong twist prior to this invention.